1. Field of the Invention
The present invention relates to an electroacoustic transducer, a process of producing the same and an electroacoustic transducing device using the same.
2. Description of Related Art
There have been proposed semiconductor devices in which capacitors capable of functioning as electroacoustic transducers such as microphones are integrated in semiconductor chips (see W084/03410, for example).
As shown in FIG. 21(e), such a capacitor is composed of an oscillation film 82 serving as one electrode of the capacitor which film is formed on a semiconductor substrate 81 having a cavity 81a, a support portion 83 of a silicon nitride film for ensuring a cavity 84a in a region corresponding to the cavity 81a of the semiconductor substrate 81, a polysilicon film 85 serving as another electrode of the capacitor formed to extend from above the support portion 83 over a part of the cavity 84a and an insulating film 87 formed on the polysilicon film 85 to substantially cover the cavity 84a with a small hole 87a above the cavity 84a. 
This capacitor is produced by the following process with connection to FIGS. 21(a) to 21(e).
First, as shown in FIG. 21(a), a diffusion layer to be the oscillation film 82 which is one electrode of the capacitor is formed on a top surface of the semiconductor substrate 81, and then, the support portion 83 is selectively formed of a silicon nitride film in a desired shape on the diffusion layer.
Subsequently, as shown in FIG. 21(b), a PSG film 84 is buried to have the same surface level as the support portion 83, on a part of the resulting semiconductor substrate 81 in which part the support portion 83 does not exist and the diffusion layer is exposed.
Next, as shown in FIG. 21(c), a polysilicon film 85 to be the other electrode of the capacitor is formed both on the PSG film 84 and on the support portion 83. At this time, the polysilicon film 85 is formed to expose a part of the surface of the PSG film 84.
Subsequently, as shown in FIG. 21(d), insulating films 87 are formed on the top surface and a bottom surface of the resulting semiconductor substrate 81. A small hole 87a is formed in the insulating film 87 on the top surface of the semiconductor substrate 81 and an opening 87b is formed in the insulating film 87 on the bottom surface of the semiconductor substrate 81.
Thereafter, as shown in FIG. 21(e), a cavity 84a is formed between the diffusion layer and the polysilicon film 85 by etching the PSG film 84 via the small hole 87a while the bottom surface of the semiconductor substrate 81 is etched until the diffusion layer is exposed, thereby to form an opening 81a. Thus the oscillation film 82 is completed.
In the above-described capacitor, the oscillation film 82 which is one electrode of the capacitor is formed inside at a certain distance from the surface of the resulting semiconductor substrate 81. The polysilicon film 85 which is the other electrode of the capacitor is formed on the surface of the resulting semiconductor substrate. With this construction, a sound wave (acoustic signal) input from the opening 81a oscillates the oscillation film 82, thereby changes the distance between the oscillation film 82 and the polysilicon film 85 which are the electrodes of the capacitor and further changes the capacitance of the capacitor. Thus generated is an electric signal equivalent to the acoustic signal.
However, the capacitor with the above-described structure has the problem of difficulty in controlling the thickness of the oscillation film 82 since the oscillation film 82 which is one electrode is formed through thinning the semiconductor substrate 81 by etching.
On the other hand, proposed is a capacitor which provides an easy control of the thickness of the oscillation film by having two electrodes on a semiconductor substrate, though this capacitor does not function as an electroacoustic transducer but functions as a pressure sensor for detecting pressure from the outside (see Japanese Unexamined Patent Publication No HEI 4(1992)-127479).
As shown in FIG. 22, a capacitor of this type is provided with a p-type diffusion layer 92, which is one electrode of the capacitor, formed on a n-type silicon substrate 91, a support layer 94 formed on the p-type diffusion layer 92 with intervention of an oxide film 93, and a polysilicon film 96, which is the other electrode of the capacitor, formed on the support layer 94 with intervention of an oxide film 95. The oxide film 95 is formed to completely cover the support layer 94 and ensure a cavity 94a in the support layer 94. A plurality of small holes 95a are formed in the oxide film 95 above the cavity 94a. The p-type diffusion layer 92 and the polysilicon layer 96, which are the electrodes of the capacitor, are connected to different wiring layers 97 and 98, respectively.
This capacitor is produced by the following process.
First, the p-type diffusion layer 92 is formed by impurity implantation at a high concentration into the surface of the n-type silicon substrate 91. Thereafter, the resulting silicon substrate 91 is entirely covered with the oxide film 93, on which the support layer 94 of polysilicon is formed in a plateau shape. The support layer 94 is entirely covered with the oxide film 95. A plurality of small holes 95a are formed in the oxide film 95. Through these small holes 95a, the polysilicon of the support layer 94 is partially etched away so as to form the cavity 94a. 
Further, a polysilicon film 96 is grown to cover the oxide film 95 by CVD method and seal the cavity 94a. The polysilicon film 96 is patterned by photo-etching to form the other electrode of the capacitor above the cavity 94a. The sealed pressure in the sealed cavity 94a at this time is a reference pressure for pressure detection.
Subsequently, another oxide film 99 is formed on the polysilicon film 96 and openings are formed in the oxide film 99 above the polysilicon film 96 and the p-type diffusion layer 92. A conductor film is formed and patterned to make the wiring layers 97 and 98 which are connected to the p-type diffusion layer 92 and the polysilicon film 96, respectively, via the openings.
In this pressure sensor, the polysilicon film 96 on the cavity 94a forms a diaphragm as an elastic member. When the polysilicon film 96 is distorted by external pressure, the pressure is detected or measured by comparing a change in electrostatic capacity between the p-type diffusion layer 92 and the polysilicon film 96 with electrostatic capacity corresponding to the reference pressure.
In this pressure sensor, however, since the polysilicon film 96 which is the other electrode of the capacitor is formed after the cavity 94a is formed, the polysilicon film 96 is warped toward the semiconductor substrate 91 and a sufficient tension cannot be ensured. If the tension of the polysilicon film 96 is extremely low, the oxide film 95 comes in contact with the p-type diffusion layer 92 which is one electrode of the capacitor. For this reason, if this pressure sensor is applied to a capacitor for generating electric signals equivalent to acoustic signals, frequency characteristics are limited within a certain range. Accordingly sufficient acoustic characteristics cannot be obtained, and electric signals equivalent to acoustic signals themselves cannot be generated. Therefore, the capacitor cannot be applied to an electroacoustic transducer such as a microphone or the like.
Further, since the cavity 94a is completely sealed with the polysilicon film 96, the cavity 94a swells if the external pressure becomes lower than the pressure in the cavity 94a, and the cavity 94a shrinks if the external pressure becomes higher than the pressure in the cavity 94a. Thus the acoustic characteristics deteriorate.